1. Field of the Invention
This invention relates to an apparatus for stretching and folding an extension jib in wheeled type cranes such as truck cranes, rough terrain cranes and the like.
2. Description of the Prior Art
Jibs to be attached to telescopic booms of wheeled cranes can be classified as truss type cantilever jibs with a truss construction and suspension-rod-supported compression column type jibs (hereinafter referred to simply as "a truss type jib or a compression column type jib" for brevity), of which the compression column type jib is accepted as being advantageous from the standpoint of cost and weight. On the other hand, with regard to folding the extension jib, there is a down-folding type which holds the folded jib on the lower side of the boom and a side-folding type which holds the folded jib on one lateral side of the boom. The down-folding type has a problem that the range of vision of the operator especially the side view range is narrowed by the overlapped boom and jib during travelling operation of the vehicle. The range of sight can be broadened by raising the height of the crane which however gives rise to other problems in the travelling operations. The side-folding type has a broader view field of the operator in the lateral direction but it has an inherent problem in that the increase of dimension in the lateral direction makes it difficult to turn the machine on narrow tracks.
In order to secure at least a sufficient side view and maneuverability in a limited space, it has been the conventional practice to employ down-folding in the case of the compression column type jib to guarantee maneuverability in narrow spaces, and to employ side-folding in the case of the truss type jib to secure a sufficient side view range. In any event, there has been no fundamental solution to either one of the problems mentioned above.
When stretching and folding the jib, the side-folding type requires ample space on one side thereof since the jib has to be turned 180.degree. in a horizontal plane. On the other hand, it is extremely troublesome to pass the main hook through the jib foot portion when stretching out the under-folding type jib.